Crystal and chemical anisotropy effects in AE2ZnN2, (AE = Ca, Sr, Ba) from ab initio

In the tetragonal ternary nitrides AE(2)ZnN(2) (AE = Ca, Sr, Ba), Zn has a rare linear coordination with nitrogen (N-Zn-N) along the c axis and N is located in an AE(5)Zn-like octahedron. Such features lead to original anisotropic crystal and chemical bond effects addressed herein with DFT-GGA based methods accounting for the cohesive energies, the energy volume equations of state EOS, the elastic constants and the properties of chemical bonding. Along AE = Ca, Sr, Ba an increasing overall compressibility under hydrostatic pressure is inferred from the respective EOS's, but the elastic constants C-ij exhibit highly anisotropic features with large magnitudes of C-33 along the tetragonal c axis versus smaller C-11 = C-22 along a, b. Decreasing C-33-C-11 along the title compounds signals decreasing anisotropy. The three ternaries are calculated as small gap insulators in agreement with experimental results of conductivity. The chemical bonding characterizes strong Zn-N bonds versus weaker AE-N thus reflecting the crystal anisotropy. (C) 2014 Elsevier Masson SAS. All rights reserved.